In part because individual judgment is not accurate enough or consistent enough, cognitive diversity is essential to good decision making. The positive case for diversity, as we’ve seen, is that it expands a group’s set of possible solutions and allows the group to conceptualize problems in novel ways. The negative case for diversity is that diversity makes it easier for a group to make decisions based on facts, rather than on influence, authority, or group allegiance. Homogeneous groups, particularly small ones, are often victims of what the psychologist Irving Janis called “groupthink.” After a detailed study of a series of American foreign-policy fiascoes, including the Bay of Pigs invasion and the failure to anticipate Pearl Harbor, Janis argued that when decision makers are too much alike—in worldview and mind-set—they easily fall prey to groupthink. Homogeneous groups become cohesive more easily than diverse groups, and as they become more cohesive they also become more dependent on the group, more insulated from outside opinions, and therefore more convinced that the group’s judgment on important issues must be right. These kinds of groups, Janis suggested, share an illusion of invulnerability, a willingness to rationalize away possible counterarguments to the group’s position, and a conviction that dissent is not useful.

In the case of the Bay of Pigs invasion, for instance, the Kennedy administration planned and carried out its strategy without ever really talking to anyone who was skeptical of the prospects of success. The people who planned the operation were the same ones who were asked to judge whether it would be successful or not. The few people who voiced caution were quickly silenced. And, most remarkably, neither the intelligence branch of the CIA nor the Cuban desk of the State Department was consulted about the plan. The result was a bizarre neglect of some of the most elemental facts about Cuba in 1961, including the popularity of Fidel Castro, the strength of the Cuban army, and even the size of the island itself. (The invasion was predicated on the idea that 1,200 men could take over all of Cuba.) The administration even convinced itself that the world would believe the United States had nothing to do with the invasion, though American involvement was an open secret in Guatemala (where the Cuban exiles were being trained).

The important thing about groupthink is that it works not so much by censoring dissent as by making dissent seem somehow improbable. As the historian Arthur Schlesinger Jr. put it, “Our meetings took place in a curious atmosphere of assumed consen sus.” Even if at first no consensus exists—only the appearance of one—the group’s sense of cohesiveness works to turn the appearance into reality, and in doing so helps dissolve whatever doubts members of the group might have. This process obviously works all the more powerfully in situations where the group’s members already share a common mind-set. Because information that might represent a challenge to the conventional wisdom is either excluded or rationalized as obviously mistaken, people come away from discussions with their beliefs reinforced, convinced more than ever that they’re right. Deliberation in a groupthink setting has the disturbing effect not of opening people’s minds but of closing them. In that sense, Janis’s work suggests that the odds of a homogeneous group of people reaching a good decision are slim at best.

One obvious cost of homogeneity is also that it fosters the palpable pressures toward conformity that groups often bring to bear on their members. This seems similar to the problem of groupthink, but it’s actually distinct. When the pressure to conform is at work, a person changes his opinion not because he actually believes something different but because it’s easier to change his opinion than to challenge the group. The classic and still definitive illustration of the power of conformity is Solomon Asch’s experiment in which he asked groups of people to judge which of three lines was the same size as a line on a white card. Asch assembled groups of seven to nine people, one of them the subject and the rest (unbeknownst to the subject) confederates of the experimenter. He then put the subject at the end of the row of people, and asked each person to give his choice out loud. There were twelve cards in the experiment, and with the first two cards, everyone in the group identified the same lines. Beginning with the third card, though, Asch had his confederates begin to pick lines that were clearly not the same size as the line on the white card. The subject, in other words, sat there as everyone else in the room announced that the truth was something that he could plainly see was not true. Not surprisingly, this occasioned some bewilderment. The unwitting subjects changed the position of their heads to look at the lines from a different angle. They stood up to scrutinize the linos more closely. And they joked nervously about whether they were seeing things.

Most important, a significant number of the subjects simply went along with the group, saying that lines that were clearly shorter or longer than the line on the card were actually the same size. Most subjects said what they really thought most of the time, but 70 percent of the subjects changed their real opinion at least once, and a third of the subjects went along with the group at least half the time. When Asch talked to the subjects afterward, most of them stressed their desire to go along with the crowd, It wasn’t that they really believed the lines were the same size. They were only willing to say they were in order not to stand out.

Asch went on, though, to show something just as important: while people are willing to conform even against their own better judgment, it does not take much to get them to stop. In one variant on his experiment, for instance, Asch planted a confederate who, instead of going along with the group, picked the lines that matched the line on the card, effectively giving the unwitting subject an ally. And that was enough to make a huge difference. Having even one other person in the group who felt as they did made the subjects happy to announce their thoughts, and the rate of conformity plummeted.

Ultimately, diversity contributes not just by adding different perspectives to the group but also by making it easier for individuals to say what they really think. As we’ll see in the next chapter, independence of opinion is both a crucial ingredient in collectively wise decisions and one of the hardest things to keep intact. Because diversity helps preserve that independence, it’s hard to have a collectively wise group without it.